1. Field of the Invention
The present disclosure relates to bearings, and more particularly to bearing supports for applications such as rotors in gas turbine engines.
2. Description of Related Art
A variety of bearings are known for use in supporting rotating components. For example, in gas turbine engines, the spools are supported by bearings for rotation of rotor blades in the compressor and turbine. Over the wide range of operational speed of a gas turbine engine, or other systems with wide ranges of operational speed, it can be beneficial to include mechanical equivalent spring stiffness to the bearing supports to optimize the rotor critical speed system and also to include damping to the spring to reduce rotor radial excursion as it passes through these critical speeds. For example, during startup of a gas turbine engine, the shaft and bearings may pass through two or more critical rotor natural frequencies (called critical speeds). If one or more of these critical speeds presents in the operational speed range, it could damage the engine. Radial springs can be provided to tune these critical speeds outside of the operational speed range.
One drawback to conventional radial springs is that if the springs are too soft, they can contribute to hung-start. Hung-start results from asymmetric temperature distribution on both rotating and non-rotating components in a gas turbine engine. For example, in an auxiliary power unit (APU) with the air inlet door closed after shut-down, natural convection gives rise to thermal gradients between the top and bottom of the engine. The thermal gradients described above can bend the rotor against the soft springs. During warm restart, i.e., restarting the engine before the engine has completely cooled, the bent state of the rotor can cause an increase in rotor imbalance, potentially leading to failure to accelerate. Hung-start occurs when the level of rotor deflection exceeds a threshold causing impeller and turbine blades to rub against their shrouds together with heavy rub of seal teeth or bristles against their corresponding runner or land. Rotor-rubbing can drastically increase drag torque, and as a consequence the starter may not be able to provide sufficient energy to overcome aero-mechanical drags. In such a case, the rotor fails to accelerate and eventually must be shut down by the control system.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for techniques to support rotors that allow for improved warm restart. The present disclosure provides solutions for these problems.